In the 300s and 400s AD
many Christians turned to believing in Manichaeism, a form of Christianity. This was started in
the late 200s AD by a man named Mani, who lived in the Sassanian
Empire and was very much influenced by Zoroastrianism.
Manichaeans believed that the world was divided into the forces of Good
and the forces of Evil, and God was the leader of the good side and
the Devil was the leader of the bad side. (Does this remind you of the
plot of the movie Star Wars? Star Wars has strong Manichaean tendencies).
This is a lot like the old Zoroastrian belief that the world is divided
between the Truth and the Lie.

In 301 AD the Roman Emperor Diocletian
began persecuting the Manichaeans and succeeded in pretty much wiping
out Manichaeanism in the Western Mediterranean and Europe. Some Manichaeans
were killed, and many others moved to the Sassanian
Empire. Others converted to Christianity. But there remained into
the 400s AD a strong strain of Manichaeanism among some African Christians.
The great African theologianAugustine,
for instance, was a Manichaean first, and only later abandoned Manichaeanism
for Catholicism.

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The problem with this idea, from the point of view
of Catholics around the Mediterranean, was that there was only supposed to be one God, who was
all-powerful. If God and the Devil had the same amount of power, or
even similar amounts of power, then there were really two gods, a good
one and a bad one. If God was all-powerful, then why didn't he just
kill the Devil and get rid of evil in the world? People have been struggling
with this question for a long time, and don't have any definite answers,
but Catholics knew that the answer could not be that there were two
gods anyway.

When thousands of Manichaean believers moved to the Sassanian Empire in the 300s AD, they kept their Manichaean beliefs there, and indeed they convinced many more people to join them, so that by the 600s when the Arabs conquered the Sassanian Empire, most ordinary people there were either Manichaeans or some other form of Christian. Gradually, over the next hundred years, most of these Christians converted to Islam.